Massachusetts investigators have developed what they call "mounting evidence," bolstered by "forensic hits," that points to the possible involvement of both Tamerlan Tsarnaev and his younger brother Dzhokhar in a gruesome, unsolved triple homicide in 2011, law enforcement officials told ABC News.

The officials cautioned that until more definitive DNA testing is complete, it is still too early to consider bringing an indictment against the younger of the two brothers, who officials said has admitted his role in the Boston Marathon bombings that killed three and injured 260 more on April 15. Tamerlan was killed in a shootout with police days after the Marathon bombing attack, but Dhzokhar survived and was captured.

Tamerlan was buried in secret, but a source familiar with the case confirmed for ABC News a report by The Boston Globe that said the 26-year-old was buried in Virginia.

In the wake of the Marathon bombings, Middlesex County began to probe a link between the elder Tsarnaev and Brendan Mess, one of the three men killed in the gruesome slaying on Sept. 11, 2011. Officials said Mess and two men were found in a Waltham residence with their throats slit and their bodies covered with marijuana. Tamerlan and Mess were once roommates and did boxing and martial arts training together.

Now law enforcement officials tell ABC News that some crime scene forensic evidence provided a match to the two Tsarnaev brothers. The officials also said records of cell phones used by the Tsarnaevs appears to put them in the area of the murders on that date. Several officials confirmed the new findings but declined to be identified because they are not authorized to comment on the ongoing investigation.

Through a spokesperson, the newly-appointed Middlesex District Attorney, Marian Ryan, declined to comment on the brothers' possible involvement in the 2011 murder. Miriam Conrad, one of the attorneys representing Dzhokhar Tsarnaev in the bombing case, also declined to comment through her legal assistant.

The savage 2011 murders unfolded on a quiet dead-end street on a balmy night.

Mess and his friends, Erik Weissman, 31, and Raphael Teken, 37, had ordered dinner from Gerry's Italian Kitchen at 8:54 p.m., but when a delivery woman arrived twenty minutes later there was no answer at the door and no one answered a call to Weissman's cell phone, from which the order was placed.

The bodies were discovered the next day. Former Middlesex District Attorney Gerry Leone said at the time that the murders were "graphic." Other investigators called it perplexing.

Roughly seven pounds of marijuana was dumped on the bodies and $5,000 in cash was left behind. Neighbors said they did not hear any signs of trouble – even with open windows – and there was no forced entry. In a 2011 interview, Leone said investigators theorized there had been more than one person at the scene of the murders based on "many factors," but no suspects were identified.

From there the case went cold, until photos of the Boston Marathon suspects were released and family and friends of the Waltham victims recognized them and remembered Tamerlan's strange behavior after the murders. He did not attend his friend's funeral and vanished from the martial arts gyms where the men had sparred together.

Then, Gerry's Italian Kitchen became a focal point again on April 24, nine days after the Marathon bombing, after investigators removed two Planet Aid charity donation bins from a parking lot it shares with a neighboring grocer. A driver had discovered discarded fireworks inside and law enforcement sources told ABC News the gunpowder had been removed from the cartridges.

It is unclear how the Tsarnaev brothers were associated with Gerry's Italian Kitchen, if at all, but looking back, multiple residents of Waltham and Watertown remember Tamerlan Tsarnaev delivering food to their homes and Tsarnaev family members have told reporters the brothers worked as pizza deliverymen. The eatery's management, however, steadfastly denied that either Tsarnaev brother worked there.

Scott Wood, a jiu jitsui trainer who befriended and trained Mess at a martial arts studio in Vermont, said he always believed whoever the killer or killers were, they got in Mess' home "under the guise of being a friend."

"Brendan was a tough, tough kid, a strong kid. It wouldn't have been easy to take him out like that," Wood told ABC News.

Wood and other friends met Tamerlan with Mess at a June 2011 fight event. Mess introduced Tamerlan as "Tam" but fighters also recognized him from his Golden Glove victories and called him "Champ." Tamerlan did not drink and was very quiet that night.

That was the last time Wood saw his friend alive.

As detectives probe the Waltham connections, Middlesex County prosecutors are also busy building a case against Dzhokhar Tsarnaev in connection with the cold-blooded killing of MIT Police Officer Sean Collier after the Marathon bombings.

Collier was shot five times, allegedly by the Tsarnaev brothers, as he sat in his cruiser, just days before he would have become a Somerville Police Officer. Collier's murder will be prosecuted by state prosecutors while the Marathon bombing will be tried in federal court.

Collier's murder was followed by a carjacking that spawned a wild, high-speed chase that ended with bombs exploding and bullets flying on a street corner in Watertown. Tamerlan Tsarnaev was pronounced dead at Beth Israel Hospital later that night. His brother escaped but was found badly wounded in a Watertown man's boat blocks from the gun battle the following evening.

Dzhokhar Tsarnaev is being held in the infirmary at Fort Devens federal prison.